Eighteen years after the last American forces withdrew from Vietnam, the United States still maintains an economic embargo against its old enemy. The reason: Despite repeated U.S. requests for information about 2,248 missing American servicemen, U.S. officials say Hanoi has refused to hand over surviving prisoners, the remains of deceased soldiers or documents that could explain their fate. While many Americans long have backed the embargo as a way to force Vietnam to hand over more information, the call of economic opportunity in Asia, including Vietnam, is eroding that support. That leaves the Clinton administration walking a fine line between honoring the demands of the friends and relatives of missing servicemen and helping American businesses compete in the burgeoning Asian marketplace.